One of the largest healthcare mergers in the country went live on Sunday. Barts Health is the new name for the joining together of three London-based NHS trusts: Barts and the London NHS trust, Newham university hospital trust and Whipps Cross university hospital NHS trust. It is now one of the largest NHS trusts in England.

Our merger strategy was carried out with consultants Ernst & Young and we have drawn three main lessons from the financial and cultural challenges of bringing these trusts together over the course of more than a year. Many other NHS trusts are also facing mergers if they are to meet the government's target of all hospitals achieving foundation trust status. They may be interested in what we have learned.

The need for a clear vision about the benefits of such a fundamental and potentially disruptive change is paramount, backed up by robust data. Many mergers, including ours, have a clear financial imperative. In our case, the two smaller trusts had no viable independent future. Our analysis showed that the merger is expected to deliver £9m in back office savings alone in its first year. But the focus of our vision was to create a world-class trust, able to stand its ground with the other big academic healthcare providers in London, such as Guy's and St Thomas' and University College London hospitals.

The merger brings security for locally provided core services, such as urgent care and maternity, while increasing access across the whole of north-east London to the highly specialist services provided by Bart's and the London, such as chemotherapy.

There was a need to invest heavily in engaging and communicating with staff, patients, doctors and user groups. Despite all the technical considerations, the most difficult challenge we faced was cultural. Without a compelling vision, people are unlikely to support a proposal that will inevitably be disruptive. Over the last year, we have engaged with 10,000 clinicians, patients and the public in the east London boroughs of Tower Hamlets, Newham, Waltham Forest and beyond. They were sufficiently behind our proposals and our commissioners were able to persuade the Co-operation and Competition Panel for the NHS – which had expressed concern about a reduction in patients' choice of provider – that the merger offered the only practical solution to the challenges facing the hospital trusts in east London.

We also learned that you have to be realistic about what can be achieved through a merger and in what timescale. We developed a three-phase approach over three years: integration, improvement and transformation.

Integration will ensure that there are no lapses in governance during the transition and that "business as usual" continues without disruption. Patient safety has to come first.

Service improvement will ensure that all services are improved to the level of the "best in trust". All three trusts excel in some areas and the priority will be on inculcating the most effective practice across Bart's Health.

The final phase is transformation. Bart's Health will develop a new approach to service delivery, such as home testing and diagnostics, virtual wards and electronic consultations. The process of integration must, however, start long before day one. Whipps Cross has already been outsourcing to Newham its non-emergency, routine operations such as hip replacements – Newham has become the merged trust's in-house team for elective work. Unless the process starts early, the integration exists only on paper and not in the minds and lives of the staff involved.

Considered on their own, effective planning, properly conducted stakeholder engagement, a service improvement focus and understanding transformation opportunities does not guarantee that the expected benefits of a major merger will be delivered. But without them, failure is almost certain.